Ionic Liquid Lubricants for Space Applications

Abstract: Lubrication is critical in order to achieve high efficiency and reliability of machine elements such as gears, bearings, and other moving mechanical assemblies (MMA). In space applications, tribological properties of lubricants are growing more important. As international space agencies are converging on the goal of establishing permanent human presence on the Moon and eventually on Mars, there is a need for radical improvements in many aspects of space exploration technology, including space tribology. Earth-orbiting satellites rely on MMA such as gyroscopes, antenna pointing mechanisms, and solar array drives. These MMA operate under lightly loaded conditions, but are subjected to high vacuum (<10-3 Pa) and the influence of space weather. Space exploration missions on the other hand, such as remotely operated vehicles used for in-situ Lunar or Mars exploration rely on different types of MMA. In these robotic systems, electromechanical actuators are being used extensively to provide controlled motion. Gears and bearings in these actuators operate in a different space environment compared to satellites, under heavily loaded contact conditions. In either case, restricted lubricant supply increases the risk that MMA operates in the harsh boundary lubricated regime, with the consequent risk of high friction, high wear, or in the worst case seizure. Fluids such as perfluoroalkyl polyethers, or multiply alkylated cyclopentanes (MAC) have a significant heritage in space because of their low vapor pressure. They are currently employed as lubricants in a wide range of space applications, as they meet high demands on resistance to vacuum outgassing. Unfortunately, these large molecules are susceptible to degradation under boundary lubrication conditions, and conventional performance improving chemical additives are incompatible with these unusual lubricant fluids.  Ionic liquids (ILs) are synthetic fluids that are composed of anions and cations. The resulting ionic interaction enables the substance to have low vapor pressure with relatively low molecular weight. For this reason, ILs have been advocated as one of the candidate lubricants for space applications. When employing ILs as lubricants, the ionic charge provides Coulombic interaction with surfaces to enable the formation of a lubricating boundary film. This is an important part of the IL lubricating mechanism, but successful lubricant performance requires integrating the lubricant candidate into the tribosystem, taking into account operating conditions and environment. Therefore, the boundary film formation should be tunable to the application at hand. Ionic liquids are designable fluids, with properties dependent on the combination of anion and cation as well as incorporated functional groups. Based on this knowledge, this work focused on evaluating this lubrication model for space applications. In this thesis, the molecular design of an IL lubricant is described (Paper 1), and the resulting hydrocarbon-mimicking ionic liquid [P-SiSO] is evaluated in tribological experiments in boundary lubricated conditions as a base fluid (Paper 2) and as a performance additive in space lubricants (Paper 3). A test methodology is devised (Paper 4) to evaluate the ionic liquid lubricant in geared actuators used in space robotics (Paper 5). The analysis is mainly based on tribological experiments followed by surface analysis by 3D profilometry by Scanning White Light Interferometry (SWLI), Scanning Electron Microscopy (SEM), and subsurface analysis by X-ray micro-tomography (XMT). Chemical analysis by Energy Dispersive X-ray Spectroscopy (EDS) reveals the formation of a highly effective boundary film based on silicate by these ionic liquids. This thesis demonstrates the feasibility of employing ionic liquids for lubrication of moving mechanical assemblies in space applications.